This application is based on Japanese Patent Applications No. 2001-115821 filed on Apr. 13, 2001, No. 2000-312073 filed on Oct. 12, 2000, and No. 11-292499 filed on Oct. 14, 1999 the contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a fuel vapor control apparatus having a canister for adsorbing fuel vapors released from a fuel tank.
2. Related Art
A fuel vapor control apparatus is used to adsorb and store fuel vapors evaporated from a fuel tank so as not to be released to the outside of a vehicle during travel of the vehicle or when the vehicle is stopped, and has a canister filled with activated carbon as an adsorbent. The fuel vapor adsorbed by the canister is desorbed and led to an intake pipe by drawing outside air from an atmosphere port of the canister by a negative pressure of an intake pipe when the engine operates, and the fuel vapor is burned along with a fuel injected by an injector.
In recent years, control on release of the fuel vapor to atmosphere is becoming rigorous. For example, in ORVR control of U.S.A. it is requested to capture all of fuel vapors from a fuel tank exhausted during refueling by a canister, not to be released to the atmosphere. Consequently, a large amount of fuel vapors has to be treated by a canister, and a canister having higher performance is demanded. The adsorption/desorption performance of activated carbon is largely influenced by temperature. The lower the temperature is, the more the adsorption amount increases. The higher the temperature is, the more the desorption amount increases. However, in the canister, the temperature changes so as to increase at the time of adsorption and so as to decrease at the time of desorption. There is consequently a problem such that the performance of activated carbon is not fully displayed. For example, in the case where the activated carbon adsorbs the fuel vapors, a capillary condensation phenomenon occurs in fine pores of the activated carbon, and the fuel vapors as gases are liquefied and adsorbed. In this case, together with the phase change from gases to liquid, adsorption heat (condensation latent heat) is generated, and the temperature increases. On the other hand, in the case where the liquefied fuel vapors are desorbed, the fuel adsorbed by purging becomes gases from the liquid while taking heat of evaporation from the surrounding, so that the temperature decreases due to the heat absorption.
In the conventional canister, due to the phenomenon, the temperature in the canister becomes higher than the ambient temperature by tens degrees at the time of adsorption. On the other hand, at the time of desorption, the temperature in the canister drops and may become equal to or below 0xc2x0 C. Particularly, at the time of desorption, a part in which the activated carbon temperature decreases due to the heat absorption reaction does not easily desorb the fuel vapors due to decrease in temperature. If the fuel vapors adsorbed cannot be completely desorbed, there is the possibility that, while the vehicle is parked, the fuel vapors diffuse in the canister and leak from the atmosphere port.
JP-A-8-42413, JP-A-60-6061, and JP-A-64-347 and Japanese Unexamined Utility Model Application JP-U-60-27813, JP-U-2-13161, JP-U-5-21158, and JP-U-58-144051 disclose a technique of heating activated carbon in a canister by heating means. The conventional techniques have, however, a problem such that activated carbon cannot be uniformly heated or a problem such that the size of the canister increases. On the other hand, the fuel vapors released from the canister are burned in an internal combustion engine. Consequently, fluctuations in density of fuel vapors or fuel vapor amount may cause poor combustion or deterioration in exhaust emission.
An object of the invention is to provide a fuel vapor control apparatus with improved performance of desorbing/adsorbing fuel vapors without increasing the capacity of a canister.
Another object of the invention is to provide a fuel vapor control apparatus capable of obtaining desired temperature in the whole area of a fuel adsorption layer in a canister.
Further another object of the invention is to provide a fuel vapor control apparatus in which an amount of heat of heating and the shape of a fuel adsorption layer are set so as to obtain desired temperature in the whole area of a fuel adsorption layer in a canister.
Further another object of the invention is to provide a fuel vapor control apparatus capable of preventing deterioration in combustion state or exhaust emission.
According to a first feature of the present invention, temperature of a fuel adsorption layer is derived by an equation expressing an amount of heat given to an adsorbent by temperature control means and a distance from the temperature control means. The amount of heat given to the adsorbent by the temperature control means and the shape of the fuel adsorption layer (distance to a part which is the farthest from the temperature control means of the fuel adsorption layer) are set so that the temperature is higher than the boiling point of the fuel and lower than the fire point of the fuel in the whole area of the fuel adsorption layer.
Concretely, when an amount of heat given to the adsorbent by the temperature control means for controlling the temperature of the fuel adsorption layer is Q(W), a distance from the temperature control means to a farthest part which is the farthest from the temperature control means of the fuel adsorption layer is X(m), and temperature of the fuel adsorption layer is T(K),
the amount Q of heat and the distance X are specified by the following relational expression (1)
T=xe2x88x92355Qxc3x97X2xe2x88x92815X+Q+298xe2x80x83xe2x80x83(1)
so that the temperature T satisfies the following condition (2) in the whole area of the fuel adsorption layer.
boiling point of fuelxe2x89xa6T less than fire point of fuelxe2x80x83xe2x80x83(2)
The relation between the temperature of the fuel and the evaporation amount is that after the temperature of the fuel exceeds the boiling point, the evaporation amount sharply increases (When the temperature of the fuel becomes equal to or higher than the boiling point, the fuel is vaporized very easily). Consequently, by setting the temperature of the fuel adsorption layer to be higher than the boiling point of the fuel, the fuel vapors liquefied and adsorbed by the adsorbent are easily vaporized and very easily desorbed. Therefore, when the amount Q of heat given to the adsorbent by the temperature control means and the shape of the fuel adsorption layer (the distance from the temperature control means to the farthest part of the fuel adsorption layer) are designed in accordance with the equation (1) so that the temperature T of the fuel adsorption layer is higher than the boiling point of the fuel, the temperature of the whole area of the fuel adsorption layer can be sufficiently controlled.
When the temperature of the fuel adsorption layer is increased too much and exceeds the fire point of the fuel, it is not desirable from the viewpoint of safety. The temperature T of the fuel adsorption layer has to be set lower than the fire point of the fuel. Consequently, by setting the amount of heat given to the adsorbent by the temperature control means and the shape of the fuel adsorption layer (the distance from the temperature control means to the farthest part of the fuel adsorption layer) so that the temperature T in the whole area of the fuel adsorption layer satisfies the condition (2), specifically, the temperature in the portion closest to the temperature control means of the fuel adsorption layer is lower than the fire point of the fuel and the temperature in the portion farthest from the temperature control means of the fuel adsorption layer is higher than the boiling point of the fuel, a canister having high desorption performance can be obtained.
The heat gradient of the canister satisfying the condition at the time of adsorption is opposite to that at the time of desorption, and the adsorption heat generated at the time of adsorption can be effectively released to the outside, so that an increase in temperature at the time of adsorption is suppressed. The canister has therefore an additional value of increased adsorption performance. Consequently, the increase in temperature at the time of adsorption is suppressed and an amount of fuel vapors which can be adsorbed increases, thereby enabling the adsorption performance to be improved without increasing the capacity of the canister. Further, since the fuel vapors do not remain in the fuel adsorption layer, the adsorbent does not easily deteriorate, and the adsorbent amount conventionally increased in consideration of the deterioration amount (by about 20%) can be decreased. Thus, the smaller size of the canister can be realized.
When the boiling point of the fuel is 318K (45xc2x0 C.) and the fire point of the fuel is 473K (200xc2x0 C.), the amount Q of heat and the distance X may be specified so that the temperature T satisfies the condition (2) in the whole area of the fuel adsorption layer.
Specifically, since the boiling points of general fuels are about 45 to 60xc2x0 C., and the fire point of a component having the lowest fire point included in the general fuel is a little higher than 200xc2x0 C., by setting the amount of heat given to the adsorbent by the temperature control means and the shape of the fuel adsorption layer (the distance from the temperature control means to the farthest part of the fuel adsorption layer) so that the temperature T lies in a range of 45xc2x0 C.xe2x89xa6T less than 200xc2x0 C. in the condition (2), the temperature of the whole area of the fuel adsorption layer can be efficiently controlled, and a canister having high desorption performance can be obtained.
The temperature control means for controlling temperature of the fuel adsorption layer may be disposed in parallel with the flow of the fuel vapors, and the fuel adsorption layer is disposed so that the whole area of the fuel adsorption layer is positioned within 25 mm from the temperature control means.
The temperature control means may be provided along a wall face of at least one of the casing wall which is in contact with the fuel adsorption layer and a partition wall for partitioning the fuel adsorption layer. Concretely, when the temperature control means is disposed along the casing wall or the partition wall, the flow of fuel vapors is not disturbed, and fabrication and assembly are easy.
The fuel adsorption layer may be partitioned so that each of partitioned parts has a flat section shape. By forming each of the adsorption layer in a flat shape with a reduced thickness, the whole area is subjected to the temperature control of the temperature control means, so that the efficiency of heat transfer can be increased.
A cross section perpendicular to the flow of the fuel vapors of the fuel adsorption layer may have an almost rectangular shape, and a partition wall for partitioning the fuel adsorption layer into a plurality of adsorption layers may be disposed in parallel with a wall face of the casing which is in contact with a long side of the rectangular shape. For example, when the partition wall is provided so as to equally divide each of short sides of the rectangle into two parts, two adsorption layers each having a flat rectangular cross section are formed, and the temperature of the whole adsorbent can be controlled with a simple configuration without finely partitioning the fuel adsorption layer.
The temperature control means may be provided integrally with a partition wall for partitioning the adsorption layer into a plurality of adsorption layers. Alternately, the temperature control means may be provided integrally with a wall of the casing for housing the fuel adsorption layer. Concretely, when the temperature control means is provided integrally with the partition wall or the casing call, the configuration becomes simpler. By controlling the temperature from the whole wall face, the efficiency of heat transfer is improved.
The temperature control means may be a heater plate for heating the adsorbent, and the heater plate may be obtained by embedding a heating element in the partition wall or the casing wall. In this case, by passing a current to the heater plate at the time of desorption to heat the adsorbent, temperature drop in associated with vaporization of fuel vapors is suppressed, and the desorption performance can be improved. Since the partition wall or casing wall itself is constructed by the heater plate as the temperature control means, the efficiency of heat transfer is high. Since the heating element is not in direct contact with the adsorbent, excellent safety is achieved.
The temperature control means may be a temperature control layer comprising a pass provided in the partition wall or the casing wall, and a medium which flows in the pass to heat or cool the adsorbent. For example, by passing the medium for heating through the vapor path at the time of desorption and passing the medium for cooling at the time of adsorption, more effective temperature control can be performed. Thus, the performance of the canister is largely improved.
By disposing the temperature control layer and the plurality of adsorbent layers alternately or in a lattice state, the temperature of the adsorbent layer is controlled from a plurality of faces, so that the efficiency of heat transfer can be increased.
According to another feature of the invention, in a fuel vapor control apparatus comprising a canister in which a fuel adsorption layer is formed by filling a casing with an adsorbent, one end of the canister being communicated with a fuel vapor path extending to a fuel tank and a purge path extending to an intake path of an internal combustion engine, and the other end of the canister being communicated with atmosphere, for temporarily adsorbing and storing fuel vapors released from the fuel tank into the fuel vapor path by the fuel adsorption layer, when the internal combustion engine operates, desorbing the fuel vapors, and transmitting the fuel vapors via the purge path into the intake path, a cross section perpendicular to the flow of the fuel vapors of the fuel adsorption layer is formed in a flat, almost rectangular shape, and temperature control means for controlling temperature of the adsorbent is disposed along the casing wall face which is in contact with a long side of the rectangular shape.
The fuel adsorption layer is not partitioned but is formed so as to have a section of a flat, almost rectangular shape having high efficiency of heat transfer, and the temperature control means is disposed along the face of the large area, thereby enabling the whole adsorbent to be heated or cooled with reliability. With the configuration as well, a small, high-performance fuel vapor control apparatus with improved adsorption/desorption performance can be obtained.
According to further another feature of the invention, in a fuel vapor control apparatus having temperature control means for controlling the temperature of an adsorbent as any of the above-described fuel vapor control apparatus, control means is provided for calculating a purge fuel amount on the basis of opening of a purge valve provided for the purge path and a detection result of an HC concentration sensor, and controlling the opening of the purge valve and operation of the temperature control means so that the purge fuel amount lies in a predetermined range.
The control means calculates an amount of purge fuel which flows into the intake pipe on the basis of purge flow rate known from the opening of the purge valve and fuel vapor concentration detected by the HC concentration sensor, and controls the opening of the purge valve so that the purge fuel amount lies in a predetermined range. When a predetermined purge fuel amount cannot be obtained by the opening of the purge valve, the operation of the temperature control means is started to promote or suppress desorption, thereby enabling the amount of fuel vapors flowing into the intake pipe to be controlled to be within the predetermined range. Thus, fluctuation in air-fuel ratio can be prevented and poor combustion and deterioration in exhaust emission can be prevented.
The temperature control means may be heating means for heating the adsorbent, and the control means may stop operation of the temperature control means when an amount of remaining fuel in the fuel tank becomes equal to or smaller than a predetermined value.
When the temperature control means is the heating means for heating the adsorbent, if the canister is in a heated state at the time of feeding fuel, the adsorption performance deteriorates. Consequently, when it is determined that the amount of remaining fuel in the fuel tank is smaller than a predetermined value, the heating of the temperature control means is stopped to prevent deterioration in adsorption performance. By setting the predetermined value to a value which is a little larger than the remaining amount at which the feeding of fuel is usually necessary, the canister temperature is low at the time of fuel feeding, so that the adsorption performance can be sufficiently displayed. In the case where the fuel remaining amount is larger than the predetermined amount and fuel is fed during the temperature control means operates, there is a case such that the time since the operation stop until the fuel feeding is short, and the temperature does not decrease sufficiently. In this case, however, the amount of fuel (that is, fuel vapor amount to be generated) is relatively small, and no fuel vapors remain in the canister because of the improved desorption performance, so that all of fuel vapors generated can be adsorbed.
The control means may stop operation of the temperature control means when the HC concentration detected by the HC concentration sensor or a fuel tank internal pressure becomes equal to or lower than a predetermined value.
When the HC concentration detected by the HC concentration sensor decreases to the predetermined value or lower, it is determined that the amount of fuel vapors adsorbed in the canister is small, and the operation of the temperature control means is stopped. Also in the case where the internal pressure of the fuel tank becomes equal to or lower than the predetermined value, it is determined that the fuel vapors do not flow in the canister. By stopping the operation of the temperature control means, the cost can be reduced by reducing power consumption and the like.
It is also possible to pressurize a closed space by heating the temperature control means, the closed space being formed in a path of fuel vapors extending from the fuel tank to the intake path via the canister when a purge valve provided for the purge path is closed, and determine a leak in the closed space in accordance with whether or not a pressure in the closed space detected by pressure detecting means reaches a predetermined pressure within a predetermined time. In this case, if the pressure reaches the predetermined pressure within the predetermined time, it is determined that there is no leak in the closed space. If the pressure does not reach the predetermined pressure, it is determined that there is a leak. Thus, the presence or absence of a leak can be easily determined without requiring a special configuration for diagnosis.
The closed space may be pressurized to a predetermined pressure by being heated by the temperature control means, after that, the heating is interrupted, and a leak in the closed space may be determined from a pressure drop state of the closed space detected by the pressure detecting means.
After pressuring the closed space to the predetermined pressure, the heating is interrupted, and the pressure drop state is monitored. With the configuration, not only the presence or absence of a leak but also the diameter of the leak hole, and the like can be known, and more accurate determination can be carried out.